This invention relates to optical correlators and more particularly to a method and apparatus for solving alignment and interconnect problems.
Optical correlators have existed in the past to provide an indication of correlation between a sample image and a reference image to provide information as to the correspondence between the sample image and the reference image.
One type of optical correlator is a van der Lugt image correlator which involves the utilization of a laser source, a pair of spatial light modulators, a detector and a number of optical elements for redirecting light from the laser and to provide for a Fourier transform and an inverse Fourier transform so that an optical correlation can be made.
One of the most serious problems with the implementation of a van der Lugt image correlator is the alignment of the optical pieces. It has been found that a misalignment of even a few wavelengths can cause a discrepancy in the correlation result. So highly accurate is the image correlation that a misalignment can cause one portion of the sample image to be shifted only minutely with respect to a corresponding location on the reference image. The result of a misalignment of even a small amount degrades the correlation obtained between the reference image and the sample image.
If the reference image is not aligned with the sample image then for any given area there may be no correlation, when there would be a positive correlation if the alignment were perfect. If one does not obtain a correlation where it is supposed to be, then applications such as the inspection of a semiconductor devices, analysis of mammography images and pap smears, signal identification and other applications of ontical correlation will suffer.
Moreover, if the alignment is not perfect, there may be false correlations across the extent of the sample image, yielding false results overall.
In one application in order to inspect a significant area, the correlator may analyze as many as 256/256 pixels. With correlation being determined on a pixel by pixel basis, the amount of pin outs required to interconnect all the active devices can exceed 100,000. Not only is this physically difficult with external wiring, the reliability of such a device is in question.
Both optical correlation systems and their components are well known as can be seen by U.S. Pat. No. 5,920,430 for Lens List Joint Transform Optical Correlator for Precision Industrial Positioning Systems; U.S. Pat. No. 5,619,496 for Method and Apparatus for Optical Pattern Recognition; U.S. Pat. No. 5,488,504 for Hybridized Asymmetric Fabry-Perot Quantum Well Light Modulator; and U.S. Pat. No. 5,951,627 for xe2x80x9cPhotonic FFT Processorxe2x80x9d.
However, none of the aforementioned patents address the problems of alignment and intraconnection for optical correlators.
In order to obtain near perfect alignment and to provide a simplified system for interconnecting the active devices of an optical correlator, in the subject invention all of the optical pieces and the active devices are mounted on or in a semiconductor substrate, with the optical alignment being referenced to the flat surface of the substrate. In one embodiment, the active devices are either embedded in the semiconductor substrate or mounted on top of it, with the surface of the substrate providing a datum plane from which alignment is established. Thus, for instance, prisms, polarizing beamsplitters, spatial light modulators and detector arrays are all referenced to the datum plane established by the surface of the semiconductor substrate.
Moreover all optical elements such as traditional lenses, Fourier transform lenses, or other optical elements are mounted directly to the surface of the semiconductor substrate which serves as a reference or datum plane, thus providing the alignment required.
Mounting the optical pieces on the semiconductor substrate means for instance that the output of a laser when redirected via a prism, through a beamsplitting device and imaged onto another prism from whence it is redirected to the surface of a spatial light modulator provides an accurately controllable alignment axis for the beam. Because of the alignment provided by the surface of the substrate the beam reflected by the spatial light modulator is directed back along this accurately determined optical axis where it is redirected by a reflective beamsplitter along a further accurately controlled axis where it impinges upon a second prism, there to be redirected onto the surface of a second spatial light modulator.
The accuracy with which light from the first spatial light modulator is directed onto the second spatial light modulator is indeed critical because while the first spatial light modulator carries the sample image, the second spatial light modulator carries the reference to which the sample image is to be compared.
Any misalignment between the optical axis on which the light travels from the first spatial light modulator to the second spatial light modulator severely impacts the accuracy of the correlation. This is because locations on the sample will not correspond to the corresponding locations on the reference.
Having established a mechanism by which an alignment can be preserved so that on a pixel by pixel basis the images can be compared, there is nonetheless the necessity of interconnecting the spatial light modulators to drive sources which are offchip. There is also the necessity for connecting to the detector array so that some offchip device can measure the degree of correlation. Alternatively, a correlation engine may be embedded into the substrate to which the detector must be connected.
In a further aspect of the subject invention, a mounting technique utilizes an epoxy frame, the top surfaces of which are polished flat to provide a plane parallel to the datum plane established by the surface of the substrate. This frame is used to mount optical elements above an active device and still provide accurate alignment.
In the subject invention, interconnection to the arrays of pixels which exist on the spatial light modulators and indeed to the CCD detector elements are carried through embedded electrical circuits within the substrate. This eliminates the large number of connections which would be necessary and, for a 256/256 array would eliminate external connections which could number as many as 100,000.
Not only is the internal interconnection of the active devices of the correlator simplified through the utilization of the embedded circuits within the semiconductor substrate, pathlinks can be reduced significantly.
In one embodiment, in the subject invention a so-called smart CMOS platform is provided to solve the connection problem mentioned above.
Thus in one embodiment the subject image correlator includes a silicon substrate with the following elements mounted to the surface of the substrate or embedded in it: a laser diode, a first prism, a first beamsplitter, a second beamsplitter, an input spatial light modulator, a first detector array, an inverse Fourier transform lens, second beamsplitter, and a filter spatial light modulator. In addition a Fourier transform lens is positioned between the two beamsplitter, with all the devices being integrated directly onto a silicon chip.
In one embodiment the detector array is preferably a pixilated detector array using MED pixels, where MED stands for modulator/emitter/detector. Alternatively other technologies such as silicon photodiode or CCD array technology are within the scope of the subject invention. Passive components, namely the prisms, beamsplitters and lenses, can be integrated directly into subsystems, also referenced to the surface of the substrate for convenient alignment and assembly. Alternatively, the Fourier transform lens may be replaced with a holographic lens.
Note, if the two dimensional detector arrays are replaced with linear arrays, then the correlator can be used for spectral analysis applications including voice recognition.
With the ability to provide appropriate alignment utilizing the van der Lugt correlator architecture with multiple quantum well spatial light modulators, the large increase in correlation rate between an image candidate and a reference is preserved due to the small size of the correlator and the enormous processing speeds which are achievable due to the small size. The subject system enables real time correlation of single reference images and near real time correlation with multiple reference images utilizing data delivery by the CMOS circuitry which is embedded in the substrate as well as the utilization of multiple quantum well spatial light modulators.
In summary, a high rate optical correlator is implemented on a substrate in which all of the optical devices are referenced to the flat surface of the substrate for optical alignment purposes by mounting the devices thereon. With the substrate surface as a reference point, alignment of the optical pieces is achieved to within a wavelength to eliminate the possibility of a xe2x80x9cno correlationxe2x80x9d result due to optical misalignment of the optical pieces. Additionally for the active elements, namely the laser, detector and spatial light modulators, interconnection of these devices and to drive sources is accomplished via direct coupling through the substrate so that the devices can communicate with each other through the silicon, thus to eliminate wire bonding and reduce pin count for the approximate 100,000 optical interconnects for a 256/256 array. Moreover, an epoxy frame which is milled at its top surface is used to mount an optical element over an active element for the alignment thereof.